P0446 OBD-II: Evaporative Emission System Vent Control Circuit

What Does P0446 Code Mean?

OBD-II Code P0446 is defined as a Evaporative System Vent Control Circuit Malfunction

The evaporative control (EVAP) system captures any raw fuel evaporating from the fuel storage system (e.g. the fuel tank, filler neck, and fuel cap). Under precise operating conditions—dictated by engine temperature, speed, and load—the EVAP system stores and purges these captured fuel vapors back into the combustion process. The carbon canister, which stores the raw fuel vapors, utilizes a vent valve that allows fresh air to enter the EVAP system. This displaces the fuel vapor as it's purged into the combustion process.

Code P0446 indicates that there is an improper voltage drop in the EVAP Vent Valve circuit, which can be caused by either too little or too much electrical resistance. This improper electrical condition will cause the vent valve to malfunction.

P0446 Symptoms

• Check Engine Light will illuminate
• In most cases, there are no adverse conditions noticed by the driver
• In some cases, there may be a noticeable fuel odor caused by the release of fuel vapors

Common Problems That Trigger the P0446 Code

• Missing fuel cap
• Defective Evaporative Vent Valve
• Defective or damaged fuel cap
• Distorted, damaged or cracked Fuel Tank Filler Neck
• Torn or punctured Evaporative system hose(s)
• Defective Fuel Tank Sending Unit gasket or seal
• Split or damaged Carbon Canister
• Defective or damaged fuel tank

Common Misdiagnoses

• Fuel cap
• Evaporative Purge Valve
• Evaporative Vent Valve

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog

Want to Learn More?

The EVAP system is designed not only to capture, store, and purge any raw fuel vapors that leak from areas in the Fuel Storage system, but also to run a series of self-tests that confirm or deny the operational and vapor holding ability of the system. This is an important task because at least 20 percent of vehicle-produced air pollution originates from malfunctioning Vehicle Fuel Storage systems.

There are many ways to "leak test" the EVAP system, but most perform the leak test when the vehicle is sitting (like over night) or during the initial start-up after the vehicle has been sitting over night. The EVAP system's operational performance is also tracked by the Powertrain Computer by reading the change in the oxygen sensor voltages and short term fuel trim whenever the stored vapors are released or "purged" back into the combustion process. These values should indicate that fuel is being added to the system and that the overall mixture is getting richer. The purging process occurs when the vehicle is under acceleration, which is when most vehicles require additional fuel.

P0446 Diagnostic Theory for Shops and Technicians

Some PCMs will trigger this code if the the proper level of vacuum was not reached during a leak test and the EVAP system contains a bypass valve that isolates the fuel tank from the carbon canister-vent valve assembly. This vacuum is measured and monitored by the Fuel Tank Pressure Sensor. The vacuum can be too high and for too long (stuck closed vent valve) or it can be too little or non-existent ( stuck open vent valve, a leak in the system or a defective fuel tank pressure sensor).

Here is how the evaporative leak test is performed by the Powertrain Computer:

1. When the leak test is performed, the vehicle must have been sitting for at least four to eight hours so that the engine temperature and outside air temperature are identical. There must also be between 15 and 85 percent fuel in the tank—this is to provide a baseline for the test since gasoline and diesel are volatile fluids that expand and vaporize easily with warm temperatures.
2. When the leak test initiates, the Vapor Canister Vent Valve is closed to prevent any fresh air from entering the EVAP system.
3. The Purge Valve is opened, which allows the engine to create a vacuum in the EVAP system.
4. After a specified time interval—usually about ten seconds—the Purge Valve is shut off and the vacuum level in the system is measured by the Fuel Tank Pressure Sensor.
5. Finally, a countdown initiates, which measures the rate at which the vacuum decays in the system. If the vacuum decays much faster than the specified rate or if no amount of vacuum is reached on two consecutive tests, then the Powertrain Computer will fail the EVAP system and trigger a P0446 code.

Common Tests for the Evaporative System

• Retrieve the code and write down the freeze frame information to be used as a baseline to test and verify any repair.
• Perform a careful and close visual inspection of all accessible hoses and components in the EVAP system for any signs of damage or degradation.
• Using a scan tool, pay very close attention to the Fuel Tank Pressure readings. Does the Fuel Tank Pressure Sensor work properly? If it doesn't, the system will think that no pressure or vacuum is being created when the EVAP monitor is performed when, in fact, there is a pressure/vacuum being created that Fuel Tank Pressure Sensor is unable to read. The Fuel Tank Pressure Sensor is the primary feedback sensor that the Powertrain Computer relies on for the leak test data each time the EVAP monitor is run.
• Inspect and test the fuel cap to determine how well it fits onto the Fuel Tank Filler Neck. Make sure the Fuel Cap Seal isn’t dry or cracked. If the cap will not seal or hold vacuum/pressure, then it can trigger the P0440 code.
• Verify that the Purge Valve and the Vent Valve work properly and hold vacuum for a sustained amount of time—at least thirty to sixty seconds. If either one of these valves function improperly, the system will not develop and/or hold the proper amount of vacuum.
• If all the components seem to function properly, then perform another smoke test of the entire EVAP system. This will usually root out any leaks that are hidden behind and/or under components of the vehicle. Pay close attention to the Fuel Tank Filler Neck, the Carbon Canister, and the Fuel Tank itself, especially where the Fuel Pump and Fuel Level Sending unit are located and sealed. Occasionally when a Fuel Pump is replaced, the seal isn't replaced or properly installed. This can cause small leaks in the system. You might have to remove the rear seats to further inspect and pinpoint the source of a Fuel Tank leak.